download
your Mikuni HSR is fitted with the tuning parts we found to work with the great majority of engine performance modifications. However, the large number of differing exhaust systems and cams available makes it impossible to accommodate all possible combinations with one carburetor set-up. Your HSR will almost certainly run correctly on your engine without exchanging any parts. But, if it doesn’t, you may alter its tuning to suit your engine’s needs by following this guide. Some exhaust system designs strongly interfere with carburetor tuning. For instance, it is very difficult to get smooth and responsive carburetion through the entire rpm range with straight pipes and completely open exhausts. In addition, very small volume, small diameter mufflers are often ‘seen’ by the engine as straight pipes and present the same tuning difficulties. Very long duration cams often cause relatively poor running below about 3,000 rpm, depending upon the individual cam’s intake valve closing point. Such cams cause reverse airflow out the mouth of the carburetor (often called “reversion” or “standoff”) that can be mistaken for a carburetor tuning problem. Harley-Davidson Screamin’ Eagle performance parts are proven and predictable. If you have any doubts about a particular exhaust system, air cleaner or ignition, you may substitute the Harley Screamin’ Eagle parts as a “reality check.” When re-tuning is required, it usually involves small alterations to the idle and/or main system. The following pages supply enough information to make such alterations relatively simple. Please note that there is no point in attempting to tune any carburetor unless the engine is sound and in a good state of tune. If you have any doubts about the general condition of your engine, have it checked by your dealer or an experienced mechanic before attempting to fine-tune your Mikuni.